Our present invention relates to a method of fiber optic temperature measurement and a fiber optic temperature sensor suitable for the online monitoring of the temperatures of electrical components and apparatus, especially electrical units utilized in the generation and distribution of electrical energy.
For online monitoring of units utilized in the production and distribution of electrical energy, for safety and reliability it is important to determine the temperature at critical portions of the apparatus or device or at critical components thereof. For this purpose, a potential free temperature measurement is required and, increasingly, fiber optic measuring systems have been involved in such monitoring.
Fiber grating sensor systems are known for temperature measurement and are potentially suitable for monitoring the temperatures of such equipment. Thus, for the sensitization of standard quartz glass fibers temperature measurement, microstructured refraction gratings can be xe2x80x9cwrittenxe2x80x9d in the core of the light waveguide at defined locations along the glass fiber. These gratings which are monolithically formed in the fiber, for example so called Bragg gratings, are capable of reflecting certain wavelengths of the light. The reflection wavelength is dependent, inter alia upon fiber temperature. The fiber grating sensor systems, however, have the disadvantage that the xe2x80x9cwritingxe2x80x9d of the grating requires a relatively expensive scanning method or the application of short laser pulses during the glass fiber drawing process which is also expensive.
Point sensitive systems utilizing fiber optics are also known and operate in accordance with various principles: Luminescence temperature sensors based upon the known characteristics of photoluminescence of various materials utilize the fact that the materials can be excited to emit characteristic longer wave radiation at certain spectral ranges by comparison to the excitation light. The measured parameter can thus either be the temperature-dependent change in the spectral intensity distribution or the extinction time of the luminescence.
Thermochromic temperature sensors utilize the effect that amplitude and the spectral location of light absorption is temperature dependent in solid and liquid substances. The strength of the absorption or transmission is thus a measure of the detected temperature.
Interferometric temperature sensors utilize the effect of temperature on the phase. They are highly temperature sensitive but generally the sensor region cannot be localized with sufficient precision.
Polarimetric sensors utilize the temperature dependence of birefrigence upon the phase of the lightwave. The temperature sensitivity is especially great with strongly birefringent fibers and as a consequence commercially HIBI fibers (high birefringent) are used. Such a polarimetric sensor is described in DE 196 44 85 A1, by way of example. A drawback with such sensors is that even the feed portions of the fiber leading to the sensor region are temperature sensitive and thus it is not possible to obtain point-wise temperature detection at specific locations of an electrical unit as is required.
It is, therefore, an object of the invention to provide a polarimetric fiber optic fiber optic temperature measurement process and a fiber optic temperature sensor for such a polarimetric process, whereby the sensor is of simple construction and easy to operate, permits spatially defined temperature measurements to be made with precision, and enables standard components to be utilized with the temperature sensor.
Another object of the invention is to provide a method of measuring temperature which is particularly suitable for use in the monitoring of the temperature of electrical apparatus and devices, especially for use in the production an distribution of electrical energy.
It is also an object of this invention to provide an improved temperature sensor which allows highly localized and pressure temperature arrangements to be made without the drawbacks of earlier systems.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention in a method of measuring temperature, especially at apparatus or equipment for the production and distribution of electrical energy which comprises the steps of:
(a) providing a polarimetrically effective fiber-optic glass fiber with a temperature-measurement region in which a coating has been removed from an optical fiber core and the core in this region is cemented to a glass capillary by a hardened adhesive;
(b) coupling into an end of the fiber-optic glass fiber on one side of the region light of a certain polarization state whereby polarization of the light is altered as a function of the temperature in the region; and
(c) measuring a difference in polarization of the light in the fiber on an opposite side of the region from the certain polarization state and thereby calculating a temperature in the region from the difference.
The temperature calculated in step (c) can be a temperature difference over time or the absolute temperature, i.e. a temperature of a region or a temperature differential. The difference in polarization is preferably measured by a polarimeter and the light can be coupled into the aforementioned end of the fiber optic glass fiber through a polarizer.
The fiber optic temperature sensor can comprise:
a polarimetrically effective fiber-optic glass fiber with a temperature-measurement region in which a coating has been removed from an optical fiber core and the core in this region is cemented to a glass capillary by a hardened adhesive;
means for coupling into an end of the fiber-optic glass fiber on one side of the region light of a certain polarization state whereby polarization of the light is altered as a function of the temperature in the region; and
means for measuring a difference in polarization of the light in the fiber on an opposite side of the region from the certain polarization state and thereby calculating a temperature in the region from the difference.
The temperature sensor can be a glass fiber especially sensitive to mechanical pressure and, for example, a Lo-Bi (low birefrigence) fiber.
Preferably the adhesive is an epoxy resin adhesive with a thermal coefficient of expansion of about 90xc3x9710xe2x88x926Kxe2x88x921. The preferred adhesive is a Delo(trademark) adhesive.
The glass capillary can be composed of quartz glass with a coefficient of thermal expansion of 0.5xc3x9710xe2x88x926Kxe2x88x921.
With the invention, the temperature dependent birefrigence effect is confined to a precisely defined narrow region of the glass fiber, namely that region in which the glass fiber, from which the coating has been removed, is bonded by the adhesive to the glass capillary. This region serves to produce a measured temperature value for the potential-free detection of the temperature in apparatus, devices and units utilized in the production and distribution of electrical energy, such as power transformers and/or electrical power switches and tapping units for such power transformers.
The sensor is particularly advantageous since it allows precise determination of the region at which the temperature is detected which is particularly important for such monitoring. It is only the region of the glass fiber which is surrounded by the glass capillary and at which the space between the fiber and the glass capillary is bridged by the sensor that functions as the temperature detector. This region, which can have the length of say a centimeter, allows measurement of temperature changes with high precision because of the significant changes in the polarization of the glass fiber in this region.